Air lift pumps and diffusers are in widespread use in the aeration of large bodies of water, such as, aquaculture ponds employed for the commercial production of large quantities of fish. In U.S. Pat. No. 4,917,832 for AIR LIFT DIFFUSER, assigned to the assignee of this application for patent, there is disclosed what may be generally characterized as a fine bubble air lift diffuser assembly for achieving wide area mixing with high oxygen transfer efficiencies so as to be ideal for waste water, lake management and aquaculture applications. The fine bubble air lift diffuser assembly disclosed in the '832 patent is broadly made up of an air lift tube and a diffuser spaced beneath the lower open end of the tube which diffuser has a fixed, non-rotating bubble-emitting surface, and a source of air under pressure is directed into the bottom of the diffuser to cause the air to be discharged upwardly through the bubble-emitting surface in the form of fine bubbles. The bubbles pass in an upward direction through the interior of the air lift tube so as to become intimately mixed with the water and dispersed in the form of dissolved oxygen over a substantial area of the pond. In order to suspend the air lift diffuser system in a body of water, the air lift tube is mounted in a return tube which in turn is secured to a float in order to maintain the diffuser in a vertical position within the body of water to be aerated.
It has been proposed to employ a diffuser of the type described in the '832 patent within an air lift tube which is anchored in a casing and submerged beneath a body of water. However, the tube is extended the entire length of the casing and in surrounding relation to the diffuser with a limited annular space between the air lift tube and casing for the introduction of water into the diffuser. Moreover, the air lift tube was anchored within the casing and not removable to permit maintenance and repair. In addition, it has been proposed to provide a circulator for the smaller home aquariums in which a vertical tube is embedded in the sand at the bottom of the aquarium and an air tube delivers air under pressure into the base of the vertical tube, as disclosed in U.S. Pat. No. 2,744,065 to Lacey. U.S. Pat. No. 4,549,997 to Verner discloses a device for increasing oxygen concentration in a subsurface stratum of a thermally stratified body of water when the air is introduced both at the lower end and midway up the shell.
U.S. Pat. No. 4,690,756 to Van Ry is directed more to a venturi-shaped tube in which the upper end is at the water surface and air as well as water are pumped into the lower end of the tube. However, none of these systems discloses a way of submerging an air diffuser wholly beneath the body of water and particularly an air diffuser of the type described in the '832 patent, in order to achieve the desired oxygen transfer. A particular problem encountered in submerging a diffuser assembly beneath the body of water is to achieve the necessary mass flow rate of water downwardly between the diffuser assembly and outer bore or casing for most efficient pumping action as well as to facilitate removal of the diffuser assembly to the surface when necessary for maintenance or repair.